Kaolin is a naturally occurring, relatively white clay which may be generally described as a hydrated aluminum silicate. Kaolin clay is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
As is well known in the industry, a large portion of the kaolin deposits in Georgia contains clays which are acceptable with regard to color and purity for refining into pigments for paper coatings, but which are too viscous to be processed by conventional kaolin processing methods. With regard to these viscous clays, a common practice is to avoid such clays by selective mining techniques. However, this practice can obviously create waste and environmental problems.
From a general standpoint, the kaolin clays of Georgia can be classified into two types--(1) hard, fine-grained Tertiary kaolin clays and (2) soft, coarse-grained Cretaceous kaolin clays. In some instances, kaolin clays exist which exhibit physico-chemical characteristics of both types.
Cretaceous kaolin clays of Georgia are soft, coarse-grained and contain abundant stacks and books of kaolinite, together with individual platelets of kaolinite. The coarse-grained nature of the Cretaceous kaolin clays is believed due to the recrystallization of kaolinite during weathering and/or diagenesis of the kaolin clay deposit.
Weathering of the sediments which formed the kaolin clays of Georgia has also formed very fine-grained (i.e., less than 0.2 microns) impurities such as smectite and illite. Within the kaolin clay deposits, these impurities may occur as coatings on the stacks, books and platelets of kaolinite. As is well known in the industry, poor theological properties can result when these expandable layer-silicates become concentrated within a kaolin clay.
In the prior art, certain patents deal with improving the viscosity of kaolin slurries by physically removing slimes. In these cases, the objective of removing the "slimes" is to reduce the content of "viscosity increasing" contaminants which, by virtue of their very fine particle size, concentrate preferentially in the ultrafine fractions (slimes) of a crude kaolin clay.
Brociner U.S. Pat. No. 3,464,634 teaches a method for lowering the low shear viscosity using a desliming centrifuge as the means of physically removing expandable layer minerals from kaolin. The invention entails deflocculating a high solids aqueous kaolin suspension with a deflocculant followed by high shear mixing (also pugging or kneading), dilution of the kaolin suspension and desliming in a high-speed centrifuge. Halaka et al. U.S. Pat. No. 5,128,027 teaches a method of removing mineral slimes which is accomplished by over dispersing a kaolin slurry, thereby reflocculating kaolin particles, and then allowing the kaolin flocculates to settle and separate. In the two above-mentioned patents, a prerequisite is that the clay be in a deflocculated state prior to and during the viscosity reducing operations.
Contrary to the prior art, in the present invention the crude kaolin material is mechanically worked (such as in a pug/kneading type mill) while in its natural flocculated state, that is to say in the absence of any deflocculating agent or dispersant. Pugging a crude kaolin in the solids range of 65% to 75%, preferably at least 71-74% solids and without dispersant or deflocculant added, followed by desliming centrifugation, produces clay slurries of lower viscosity than those produced by similarly deslimed clays that have been pugged in a deflocculated state.
Pugging alone has also been shown to improve the viscosity of kaolin slurries. U.S. Pat. Nos. 2,535,647 and 2,907,666 of Millman el. al. teach that mechanically working a high solids kaolin/water mixture can dramatically reduce the low shear viscosity of kaolin slurries. U.S. Pat. No. 2,535,647 states that a deflocculating agent is added during the pugging while U.S. Pat. No. 2,907,666 kneads the clay in the absence of dispersant. Millman et al. U.S. Pat. No. 3,106,476, discloses that high intensity agitation of kaolin slurries in the presence of dispersant is also helpful in improving the low shear viscosity of kaolin clays.
Other patents utilize high intensity mixing or pugging of a high solids kaolin/water mixture to reduce viscosity; however, the mixing and pugging are used in conjunction with other treatments. These patents use the synergistic effect of pugging coupled with other treatments to improve the rheological characteristics of kaolin slurries. For example, in U.S. Pat. No. 3,301,691 pugging coupled with heat treatment of the kaolin is discussed. U.S. Pat. No. 3,326,705 pugs in the presence of hydrochloric acid for viscosity reduction. Finally, U.S. Pat. Nos. 3,510,330 and 3,510,331 add urea and carboxylic acid, respectively, during the pugging stage.
The low shear viscosity of the crude kaolin clay and of the particle size fractions derived from that crude clay is one of the principal physical characteristics determining whether the clays and derived fractions will be utilized in the production of high value-added products for the paper industry. For example, see U.S. Pat. Nos. 4,105,466 and 4,030,941.
Therefore, a need exists in the industry for a method by which these crude viscous kaolin clay materials are rendered processable and ultimately useful as pigments and in paper coating applications.